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WØINK February 2009 BALUNSPage 2 of 25 TOPICS The BALUN & Antenna Problems What is a BALUN? The Current BALUN The Voltage BALUN Why & How to use a BALUN? Selecting a BALUN

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WØINK February 2009 BALUNSPage 3 of 25 TYPICAL ANTENNA PROBLEMS Radio Interference to nearby devices. Transmission line radiation. The above are due to “common mode currents” on the transmission line. Loss of power to antenna due to mismatch between coax and antenna. BALUNS can address these problems.

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WØINK February 2009 BALUNSPage 4 of 25 BALUN – A Coined Word Balun formed from BALance – UNbalance. Name suggest device converts between “Balance <> Unbalance”. BALUN is name of device that can be many things like a common mode choke, unbalance to balance transformer, and a step up or down transformer.

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WØINK February 2009 BALUNSPage 5 of 25 A Typical BALUN

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WØINK February 2009 BALUNSPage 6 of 25 Common Mode Currents How are common mode currents generated? Why are common mode currents undesirable?

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WØINK February 2009 BALUNSPage 7 of 25 Generation of CM Current From an Antenna

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WØINK February 2009 BALUNSPage 8 of 25 Problems Caused By CM Current Pattern Distortion – caused by radiation from transmission line due to common mode current on transmission line. RFI to nearby devices like TV sets and telephones. RF in the shack or transmitter location that can cause RF current to flow through the operator!

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WØINK February 2009 BALUNSPage 9 of 25 Types of BALUNS There are two general types of baluns. Current or choke type Baluns. => Eliminates “common mode currents”. Voltage type Baluns. => Provides for impedance matching of transmission line to antenna. An example is the TV antenna 300 ohm to 75 ohm matching transformer.

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WØINK February 2009 BALUNSPage 10 of 25 CURRENT BALUNS Current baluns are typically 1:1 -- Zout = Zin but can have other ratios. Primary use is for reduction of common mode currents. Can be use for unbalanced to balanced loads like dipole antennas and beams. Typical construction is ferrite beads over coaxial cable or wires on ferrite core.

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WØINK February 2009 BALUNSPage 12 of 25 Balanced Load – Balun Let Zcc be Hi-Z due to ferrite beads on cable and R1 = R2. Then R1 is not shunted by Zcc. Now E1 = E2 – balanced voltage across load due to balanced load – not balun.

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WØINK February 2009 BALUNSPage 13 of 25 Common Mode – No Balun Let Zcc be 0 ohms which is in series with Zcm. Then Icm = Eg/x / Zcm

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WØINK February 2009 BALUNSPage 14 of 25 Common Mode – Balun Let Zcc be Hi-Z due to ferrite beads on cable which is in series with Zcm. The Icm = Eg/x / Zcm + Zcc. Note if Zcm where infinite, then Icm = 0 Amps

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WØINK February 2009 BALUNSPage 15 of 25 VOLTAGE BALUNS Voltage baluns have varied input to output ratios and have true balanced or unbalanced outputs as determined by the design. Primary use is for impedance matching of typically a transmission line to an antenna. Typical construction is the use of coaxial cable or wires on a ferrite core.

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WØINK February 2009 BALUNSPage 16 of 25 1:1 Voltage Balun Example of a 1:1 unbalanced to balanced voltage balun. Balanced output is due to transformer and not the load. There is essentially no common mode current rejection.

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WØINK February 2009 BALUNSPage 17 of 25 4:1 Voltage Baluns Examples of 4:1 voltage baluns that can be connected for either balanced or unbalanced output.

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WØINK February 2009 BALUNSPage 18 of 25 WHY USE A BALUN? Baluns are used for two primary reasons. One : Eliminate “common mode current” on the transmission line! Two : Matching antenna impedance to the transmission line.

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WØINK February 2009 BALUNSPage 19 of 25 Matching Antenna Impedance matching antenna to transmission line improves power transfer and lowers SWR. Note! A high SWR on a transmission line does NOT cause the transmission line to radiate!

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WØINK February 2009 BALUNSPage 20 of 25 Balun Installation

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WØINK February 2009 BALUNSPage 21 of 25 Selecting A Balun ALWAYS use a current balun to eliminate common mode current and if matching is needed, current baluns can also have a ratio (4:1 or 9:1) as required to match antenna. Use a voltage balun only for impedance matching of antenna to transmission line. Becoming knowledgeable on how and why baluns work, will insure success.

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WØINK February 2009 BALUNSPage 22 of 25 Purchasing A Balun The description of a balun, may or may not, include that it is current or voltage balun. A 1:1 current balun is sometimes described as a choke balun. An example is the catalog description of the W2AU ferrite balun usually does not indicate that it is a voltage balun.

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WØINK February 2009 BALUNSPage 23 of 25 Make A Balun A current balun can be made by winding several turns of coax to form a inductor. Another choice to make a current balun is to place a number of ferrite beads over coax. Need More Ideas: “GOOGLE” for one that meets your needs.

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WØINK February 2009 BALUNSPage 24 of 25 REFERENCES Baluns: What They Do And How They Do It By Roy W. Lewallen, W7EL. Article in “The ARRL Antenna Compendium” Vol 1. Understanding, Building, and Using Baluns and Ununs By Jerry Sevick, W2FMI. A book published by CQ Communications.